Assuming they are doing the same amount of HD as other "HD" games, it won't be realistic or super high def

and yes, it is simple, you just need to create a stretched image to wrap around a mesh, or you can create individual textures for each portion of the mesh (this looks best if you know how to connect them at the sides)

If you are starting from scratch with no idea what you are doing or have to create the meshes as well, yes it takes a long time. However, as an HD port, they already have the meshes prepared.